1. Field of the Invention
This invention relates to a method for producing chemical device elements that are used to manipulate, store and/or treat highly corrosive products, for example, storage chambers, containers, reactors, mixers, treatment devices and devices for conveying raw or intermediate products. To ensure good corrosion resistance and in consideration of the cost of anticorrosion materials, chemical device elements usually comprise a steel support (carbon steel or stainless steel) which gives the assembly its mechanical strength and an anticorrosion metal coating based on a noble metal or on a reactive metal capable of providing a protective layer after reaction with the oxygen or with the corrosive environment concerned. For example, a material such as tantalum, tungsten, vanadium or alloys thereof, or, if conditions allow, reactive metals such as zirconium, titanium, hafnium or alloys thereof, are selected. Due to the cost of these materials, it is desirable to make the coatings as thin as possible.
This invention relates more specifically to chemical device elements comprising a thin zirconium coating having a thickness of typically less than 1 mm. It may also relate to the production of device elements internally coated with zirconium and used for storing, exploitation and/or transport of nuclear materials. We will hereinafter refer to the latter with the general term “nuclear devices.”
2. Description of Related Art
It is possible to produce chemical device elements in a number of ways, whether by “lining” the inside of the chemical device once it has been completely formed, by depositing the coating on portions already formed, then assembling them, or by depositing the coating on plate- or tube-type semi-finished products, forming said semi-finished products thus coated, then assembling the various portions thus obtained.
In the first case, the lining can be made without a connection between the support and the coating (“loose-lining”). For example, a mechanical engagement can be provide between the coating and the support, by anchoring, in a limited number of assembly points. Such a technique in theory makes it possible to use anticorrosion coatings having a thickness of several hundred microns. However, for apparatuses subjected to significant mechanical stresses, the use of a coating having a low thickness is not desirable insofar as the coating is not closely attached to the substrate and risks being weakened, or even collapsing, when the chamber is subjected to a vacuum.
In other cases, a number of techniques for attaching the coating to the support are possible. It is possible to provide several spot welds, for example by seam welding, or to closely attach the coating and the support over the entirety or a large portion of their opposing surfaces, by explosion cladding or by melting of an intermediate brazing alloy layer. If spot seam welding is performed, it is possible to encounter excessive local deformations of the coating if it is too thin. Moreover, an inadequately supported coating can collapse if the chamber is in a vacuum. If the assembly is performed by explosion cladding, it is not possible in practice, i.e. with the standard shapes of chemical device elements to be coated, to control the propagation and the effect of the shock wave if the thickness of the coating is less than one millimeter, when the plate and its coating cover large surfaces, typically several square meters. If the attachment of the coating is performed by brazing, two techniques are possible: brazing the coating onto one portion of the device already formed (as in U.S. Pat. No. 4,291,104) or brazing the coating onto a plate which is then formed (as in WO 03 097230).
The method described in U.S. Pat. No. 4,291,104 (Keifert) consists of using a coating that is clearly thinner than the support, pre-deforming it so that it follows the shape imposed by the support, and to provide suitable locations for “convolutions” which make it possible to compensate for differential expansions, and placing said coating, then attaching it by brazing, onto the support. Although such a technique is suitable for handling a coating that is relatively thin with respect to the support, it requires the use of coatings that are not excessively fragile or excessively deformable during the handling necessary for placing the coating onto its support. These coatings must therefore have an adequate thickness, of which the value is a function of the shape of the coating pre-formed before its attachment to the support, and which is typically greater than 0.75 mm.
To ensure the cohesion of the assembly with a coating having a thickness of less than 1 mm, it is possible to consider using thermal spray technologies: plasma-assisted or non-plasma-assisted hot or cold spraying. However, while these technologies make it possible to obtain both good cohesion of the assembly and low noble metal consumption, they do not provide 100% assurance of the impermeability of the anticorrosion coating thus deposited, even if its porosity level is generally lower than the percolation threshold.
It has long been known that direct welding of zirconium on steel is very difficult. Patent GB 874 271 (inventor: Alan Garlick) proposes the use of two different intermediate metal layers between the zirconium and the steel: the steel plate is coated with vanadium, and the zirconium plate is coated with niobium or titanium; the thickness of each of these coatings is on the order of one millimeter. Intermediate metals have also been used to join two plates of zirconium or zircaloy to one another (see U.S. Pat. No. 3,106,773), but the thicknesses used were much lower: on each side, a thickness on the order of 5 to 500 nm for the titanium, and on the order of 300 to 500 nm the copper was used.
In patent application WO 03 097230, the applicant showed that it was advantageous to produce chemical device elements by using steel plates or sheets which are coated with a metal anticorrosion material by melting of an intermediate brazing alloy layer, forming the plates thus coated by plastic deformation, then welding them to one another so that the element obtained confers its final shape on the device. For economical and mechanical reasons (the support must be prevented from losing its mechanical characteristics during the brazing, for example by exceeding its austenitisation temperature), the brazing temperature must be as low as possible. For this reason, a brazing alloy well suited to the desired low temperatures, which includes silver and copper, and of which the melting temperature is below 900° C., is used. However, this type of alloy cannot be used to braze a zirconium coating because the zirconium reacts with the copper and the solder to produce fragilising compounds that significantly limit the ductility of these assemblies. This loss of ductility leads to decohesion of the coating during forming by plastic deformation of the assemblies (production of a dished bottom, for example) and the chemical device thus loses its corrosion resistance.
The applicant has attempted to develop a process making it possible to obtain chemical or nuclear device elements comprising a zirconium or zirconium alloy coating, which typically has a thickness of less than 1 mm, preferably less than 0.5 mm, or even 0.3 mm and which does not have any of the disadvantages of the methods of the prior art.